In conventional marine power distribution and propulsion systems that employ full electric propulsion (FEP), fault current magnitude-time discrimination is used to enable protective switchgear to interrupt over-current faults in particular sub-circuits whilst causing the minimum practical disruption to all other sub-circuits. Such FEP systems are said to employ the “power station principle” where the aim is to adapt the power generation capacity that is on-line at any particular time to the total load that is being drawn at that time. This has the effect of maximizing fuel efficiency. The configuration of such FEP systems is normally automated to some degree by a power management system with the authority to shed load and start generators in a prioritized manner. Alternating current is distributed through the FEP systems at medium voltage (MV) to maintain compatibility with land-based systems.
An example of a conventional FEP system is shown in FIG. 1. A series of turbines T and diesel engines D are used to power individual generators G. These supply ac power to the FEP system through a medium voltage (MV) ac busbar system that is equipped with protective switchgear. The protective switchgear comprise circuit breakers and associated controls and are represented in FIG. 1 by the × symbol. Power converters PC are used to interface the MV ac busbar system to an electric propulsion motor PM that drives a propeller. Filters F are also connected to the MV ac busbar system. The MV ac busbar system is divided into a first MV ac busbar and a second ac MV busbar that are interconnected by protective switchgear. A first low voltage (LV) ac busbar is connected to the first MV ac busbar through a first transformer. A second LV ac busbar is connected to the second MV ac busbar through a second transformer. The first and second LV ac busbars are interconnected by protective switchgear. A series of unspecified large and minor loads can be connected to the first and second LV ac busbars, respectively. It will be clear from FIG. 1 that the minor loads are connected to the first and second LV ac busbars through first and second minor LV ac busbars.
Six magnitude-time discrimination levels of the FEP system are shown along the right hand side of FIG. 1. Protective switchgear is represented by the × symbol in each of the discrimination levels. For example, in discrimination level 6 protective switchgear is located between the MV ac busbar and each of the generators G. In discrimination level 5 protective switchgear is located between the MV ac busbar and each of the filters F and between the MV ac busbar and each of the power converters PC. Protective switchgear is located between the MV ac busbar and each of the transformers that are used to connect the first and second MV ac busbars to the first and second LV ac busbars, respectively. In discrimination level 4 protective switchgear is located between each of the transformers and the respective LV ac busbars. In discrimination level 3 protective switchgear is located between the first and second LV ac busbars and each of the large loads and between each of the respective feeds to the minor LV ac busbars. In discrimination level 2, further protective switchgear is located between first and second LV ac busbars and the associated parts of the minor LV ac busbars. In discrimination level I protective switchgear is located between the minor LV ac busbars and each of the minor loads.
A short circuit in any particular discrimination level of the FEP system must trip the associated protective switchgear in that level but must not cause any other protective switchgear to trip. Protective fault current levels are determined entirely by supply impedance and the protective switchgear is only able to interrupt the fault current (i.e., the current flowing in the FEP system during a fault) well after the peak fault current has passed. The fault current is therefore normally only interrupted at, or very shortly after, line current reversals.
The conventional FEP system shown in FIG. 1 has the following technical disadvantages.
The magnitude of the fault current is influenced by the number and type of generators G that are on-line on a particular point of common coupling; the lower the combined generator impedance the greater the fault current. Wide variations in prospective fault current occur and protection equipment setting may have to be continuously adjustable to guarantee fault discrimination.
The magnitude of the fault current is increased as distribution voltage (i.e. the voltage carried by the various ac busbars in the FEP system) is reduced. As the total installed power rating is increased and/or distribution voltage is reduced, the resulting fault current may exceed the capability of the available protective switchgear. Medium voltage power distribution systems may have to resort to the use of load step-down transformers and specialized insulation systems in order to allow a sufficiently high distribution voltage to be used to overcome protective switchgear limitations.
The characteristics of the generators G may vary widely in terms of time dependency and peak magnitudes of ac and dc components to aid load sharing. (Automatic Voltage Regulators (AVRs) are designed to aid load sharing.) Moreover, these characteristics are greatly influenced by the type of prime movers (diesel engine D or turbine T, for example) that is coupled to the generator and their resultant coupled governed and regulated responses may be subject to significant disparities. When a group of generators G is connected to a point of common coupling then disparities often become problematic, particularly during the switching of passive circuits such as filters and transformers and during load transients.
The FEP system is often split into multiple points of common coupling that are often referred to as “islands”. All islands may be connected together in parallel to give a single island arrangement (e.g., for single engine running) or may be separated to provide redundancy and graceful degradation of capability following equipment failures. Synchronization and load transfer between individual islands is complicated, particularly when they have different degrees of harmonic pollution and when the disparities mentioned above are present. Propulsive power is normally drawn from the islands in a Propulsion Distribution System (PDS) and other loads can be fed by islands in a Ship Service Distribution System (SSDS) whose power is usually derived from the PDS. Protective discrimination and quality of power supply are usually related by common hierarchy that extends from the largest generator G down to the smallest electrical load. Means must be provided to decouple the relatively sensitive SSDS from the potentially harmful effects of the relative robust power and propulsion equipment in the PDS. Critical electrical loads may require local high integrity power supplies of their own with dedicated power conversion and energy storage equipment in order to attain the required degree of decoupling from the PDS. These local power supplied are often referred to as Zonal Power Supply Units (ZPSU) and their energy stores are often referred to as Zonal Energy Stores (ZES).
Since the FEP system is an ac system a number of variables can affect its design. These include inter alia voltage, frequency, phase angle, power factor, point in cycle switching events, phase imbalance, integer and non-integer harmonic distortion. Because it is a complex ac system it is recognized that it is very difficult to damp the inevitable resonant modes between stray and intentional impedances that affect such a power distribution system. Once an ac distribution frequency (i.e. the frequency of the ac current carried by the various ac busbars in the FEP system) has been chosen then this will greatly influence the generator topology and ultimately places limits on the shaft speed of the prime mover. In many cases, this will adversely affect the size and performance of the generator and the prime mover.
Whilst most conventional FEP systems distribute ac current at medium voltage (MVAC), it is also known to distribute dc current at low voltage (LVDC). Although these LVDC systems derive their dc current from MVAC current supplies via current limited power electronics, they rely on dc circuit breakers (DCCB) to interrupt significant fault currents.
For example, an SSDS may use phase-controlled transformer rectifiers to derive a LVDC distribution voltage from a conventional MVAC distribution system. Parallel redundant feeders distribute the LVDC distribution voltage through switchboards that include fault current-rated DCCBs. Each ZPSU is fed from a redundant pair of these switchboards via interposing regulated power electronics and anti-backfeed diodes.
Another SSDS may use transformer-isolated back-to-back pulse width modulated (PWM) voltage source inverters (often referred to as MV/LV link converters) to derive the LVDC distribution voltage from a conventional MVAC distribution system. The LVDC is distributed using a ring main to provide redundancy then via fault current rated DCCBs to ZPSUs and other electrical loads.
Unlike in a conventional ac current distribution system, a dc current distribution system will not experience regular current line reversals. The DCCBs must therefore interrupt fault current by electromechanically causing contacts to open, thereby causing arc voltage to be generated between the contacts. The arc voltage opposes a system voltage that is the sum of the power supply voltage source that causes the fault current to flow and the inductively generated voltage that opposes any reduction in the fault current. This allows the arc voltage to reduce the fault current and eventually completely interrupt it. As the fault current approaches final interruption, the arc voltage will experience a transient increase that is known to stress components that are connected to the SSDS and which generates electromagnetic interference (EMI). This component stress is exacerbated by the summation of the DCCB transient arc voltage and the recovery of the SSDS distribution voltage that results from the interruption of the fault current that flows in the power supply voltage source. It is known to apply surge arresters and snubbers to such power distribution systems to reduce the transient arc voltages and EMI.
It is also known to use hybrid DCCBs that use a series connected combination of power electronic switching devices and electromagnetically actuated electrical contacts such that the power electronic switching devices rapidly switch off, a surge arrester and snubber moderate the resultant voltage transient and the electrical contacts are opened following the interruption of the fault current.
Linear regulator dc power supply units use a technique called “foldback” to limit regulator power device dissipation during short circuit load conditions. A foldback system typically comprises an output current limiting regulator whose reference is output voltage-dependent. If load impedance drops below a particular threshold, the initial action of the current limiting regulator is to cause the output voltage to reduce, followed by a regenerative action that serves to limit the output current and voltage to suitable low levels and limit regulator power device dissipation.